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New Scientist Live

Body heat

What mechanism in mammals provides the temperature reference point that enables humans and other animals to have such accurate control of body temperature?

“The oryx maintains a brain temperature of 39 °C, while the rest of its body is 5 °C warmer”

• Body temperature in mammals is controlled by a region of the brain called the hypothalamus. This monitors the temperature of blood flowing from the heart through the brain, as a measure of the body’s core temperature – the temperature of the heart, liver and so on. If the hypothalamus detects a move away from the normal -37 °C in humans – it coordinates a series of responses to lower or raise body temperature. If a rise is detected, it triggers sweating and dilation of the arterioles running close to the skin in order to lose heat. If the core temperature is too low, it encourages an increase in metabolic rate, constriction of the arterioles near the skin and erection of hairs on the skin to conserve heat. If body temperature continues to fall, it triggers shivering to generate heat.

Katherine Gourd, London, UK

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• The basic temperature control mechanism is a “thermostat” in the hypothalamus whose neurons fire more rapidly when they get warmer and more slowly when the temperature drops. These cells use hormones to signal changes in the basal metabolic rate; send nerve signals to initiate shivering, sweating or panting; or redistribute blood flow to and from the limbs and skin to lose or conserve heat.

The questioner suggests that mammals set a precise temperature&colon; they do not. For example, our temperature rises when we are fighting infection, which helps to kill or disable it. In fact, syphilis used to be cured by infecting patients with malaria. Several fever cycles would kill the syphilis, and the doctor could then administer quinine to treat the malaria.

During exercise, our body temperature is allowed to rise to between 39 and 40 °C. You know the body “allows” this to happen because you do not start to sweat until your body temperature reaches the new set point appropriate to the increased level of activity.

Other animals can also vary their body temperature. Under normal conditions, desert mammals maintain a temperature of around 37 °C. But when deprived of water, some species allow their bodies to warm up considerably to 39 °C or more during the day. By so doing, they save a lot of water that would otherwise have been used for evaporative cooling. At night, when the temperature falls dramatically, they allow their body temperature to fall well below the normal 37 °C set point to as low as 35 °C.

Some desert antelope, such as the east African oryx, let their body temperature rise as high as 45 °C, which would be hot enough to thoroughly addle their brains. However, their panting preferentially cools the arterial blood feeding their brains, allowing them to maintain a brain temperature of around 39 °C while the bulk of their body is about 5 °C warmer. This also saves water that would otherwise be used for cooling.

Most animals allow their extremities to cool considerably below their core temperature. This is done partly by reducing the rate of blood flow to their limbs and tail, and partly by directing the cool venous blood returning to their heart into a set of veins running close to the arterial supply. This heat exchanger cools the arterial blood flowing to the limbs, and warms the returning venous blood, thus retaining body heat and saving metabolic energy.